Semiconductor devices are made on substrates, such as silicon wafers or glass plates, for use in computers, monitors, and the like. These devices are made by a sequence of fabrication steps, such as thin film deposition, oxidation or nitration, etching, polishing, and thermal and lithographic processing.
Thin film deposition typically is performed in a low-pressure processing chamber. In chemical vapor deposition, a wafer or other substrate is loaded into a processing chamber and is exposed to a process gas under suitable conditions for deposition of the process gas or a component of the process gas in the form of a thin film on the wafer.
There are a number of different varieties of chemical vapor deposition processes, of which one of the more recently developed is referred to as atomic layer deposition (ALD) or atomic layer chemical vapor deposition (ALCVD). In atomic layer deposition, many thin film layers are deposited on the wafer in a repetitive process in which the wafer is alternately exposed to more than one process gas. Each cycle of an ALD process entails opening and closing a number of valves which control the flow to the processing chamber of process gases or a purge gas. Because each cycle is repeated numerous times, the amount of time required to generate, transmit and execute valve opening and closing commands may be a significant factor in the overall elapsed time required to complete an ALD process. The present inventors have recognized that a key to improving throughput for ALD processes lies in shortening the “refresh time” for valve control commands, where “refresh time” refers to the time required to generate, transmit and execute a command.